My UV Islands Overlap
It's unlikely that this overlap matters; assuming this model works okay in GMod, these are probably different materials that use different textures.

However.  A smart first step is to create a new UV map to work with, so that you can later bake any textures from this UV map to your new UV map.  Switch to properties view/data/UV Maps, select your UV map from the list with LMB, then hit the + button to the right of the list.  This copies that UV map to a new UV map.

Okay.  Now select your object, switch to edit mode, and select all vertices.  Open up a UV editing window; if it's set to a rendered image, change the image to something else so that you can see and edit your UV coords.  Select all verts in the UV editor ('a' once or twice or however you want.)  Hit ctrl-p to pack the islands.  This will translate and scale the UV islands in order to eliminate any overlap.  (There are other techniques you can use, but packing islands is the easiest and simplest.)

At this point, if you need to, you can bake your original textures from their original UV to your new, packed UV.